The amplitude (magnitude) A (A&gt;0) and phase .phi. of the sinusoidal signal X(t) of the form EQU X(t)=A sin(.omega.t+.phi.)
where .omega. is a known angular frequency in radian per second, are important parameters to measure in many practical measurement and control applications in the power system or the like. Many methods have been proposed for the purpose and include the zero-crossing detection (ZCD) method, the discrete Fourier transform (DFT) method, and the least square error method (LSM). The zero-crossing detection method transforms the signals into two square waves, and then measures the time difference between the zero-crossings of the two square waves which is proportional to the phase. This method suffers from phase errors clue to the hysteresis and different signal amplitudes. An article entitled "A Novel Digital Phase Meter" by Ibrahim et al in IEEE Transactions on Instrumentation and Measurement, Vol. IM-36, pp 711-716, September 1987, describes such a measurement.
The discrete Fourier transform method performs the DFT on N digitized samples of the signal X(t) which are sampled uniformly at a higher rate than the Nyquist sampling rate. The signal X(t) is converted into a causal discrete-time sinusoidal signal x(n) EQU x(n)=A sin(.omega.nT+.phi.) for n=0, 1, 2, . . . (1)
where T is a sampling period in seconds. Subsequently the DFT coefficient is obtained and using this coefficient the amplitude and phase of the signal are calculated. The method however suffers the effect of spectral leakage from the edge discontinuities arising from the beginning and ending of the samples.
In an article entitled "Phase Angle Measurement Between Two Sinusoidal Signals" by R. Micheletti in IEEE Transactions on Instrumentation and Measurement, Vol. 40, No. 1, pp 40-42, February 1991, a new algorithm for measurement of phase angle is described to be based on the least square error method (LSM). The algorithm processes digitized samples of two input signals to calculate the phase angle between them. The algorithm is a general form of measuring technique and uses any arbitrary discrete sampling frequencies and sample sizes. Computation is quite complicated.
The present invention is a new modified LSM of measuring the magnitude and/or phase angle of a sinusoidal signal. Within a wide variety of possible applications, the present disclosure describes embodiments wherein the novel method is implemented in the telephone environment.
In the field of telecommunications, a telephone subscriber's loop may contain one or more load coils which compensate capacitive and inductive impedance matching to improve frequency response for analog telephone signals. Digital services such as Integrated Services Digital Network (ISDN), Digital Subscriber Line (DSL), Asymmetric Digital Subscriber Line (ADSL) and High-bit rate Digital Subscriber Line (HDSL) require much higher bandwidth than the analog plain old telephone service (POTS) does and since load coils behave like a low pass filter with a cutoff frequency of about 4 kHZ, they are an impediment to delivery of these digital services. U.S. Pat. No. 4,486,627 issued Dec. 4, 1984 (Beeman et al) describes an arrangement for monitoring the admittance of a line during a frequency sweep. The characteristic admittance level determines the loaded or unloaded status of a transmission line. Output logic associated with detection circuits provides a digital indication of the status detected. U.S. Pat. No. 4,620,069 issued Oct. 28, 1986 (Godwin et al), on the other hand, teaches techniques for measuring the loop impedance at either one frequency or a plurality of frequencies. Changes in either the real part of the impedance or the phase angle are used to determine the loaded condition of the subscriber's loop. Godwin et al describe two methods of measuring the impedance. One method is simply to connect an analog stimulus signal to the loop and measure by analog signal processing the real part and/or the phase angle of the impedance. Another method involves the use of line circuits provided at a central office digital switch. The digital switch uses codes and therefore this is a digital measurement of the impedance. The method also uses Digital Fourier Transform for calculating magnitude and the phase angle of the impedance.
The prior art techniques for load coil detection suffer from inaccurate measurement, cumbersome procedure, slow response, etc.